1. Technical Field
The present invention relates to a method for controlling a brake system of a motor vehicle with all-wheel drive, in particular for controlling a regenerative brake system with a number of friction brakes and an electro-regenerative brake. The invention further relates to a regenerative brake system for an all-wheel driven motor vehicle.
2. Description of the Related Art
The purpose of regenerative brake systems in motor vehicles involves storing at least part of the energy produced during braking in the motor vehicle and re-using it for the drive of the motor vehicle. This provision allows reducing the energy consumption of the vehicle in total, increasing the efficiency, and thus rendering its operation more economical. To this effect, motor vehicles with a regenerative brake system generally include different types of brakes, also called brake actuators.
Typically, this arrangement employs hydraulic friction brakes, as known from customary motor vehicles, and an electro-regenerative brake. Like in conventional friction brakes, the brake pressure for the friction brakes is generated by means of a brake pressure generating means or by way of the brake pedal movement, respectively. The electro-regenerative brake is generally configured as an electric generator, generating at least part of the total brake output. The produced electric energy is supplied into a storage medium such as an on-board battery, or is fed back, respectively, and is reused for driving the motor vehicle by way of an appropriate drive.
Regenerative brake systems can be designed as so-called serial regenerative concepts where the component of the brake torque, which is produced by the generator, is as high as possible. In contrast thereto, parallel or so-called residual-moment-based regenerative concepts are known as well, where the brake torque is distributed in predefined ratios to the brake actuators. Mixed concepts of these two brake concepts are likewise known. It is common to all systems that they use several brake actuators for simultaneous braking at least in some ranges of the brake torque to be generated so that the total deceleration is composed of the deceleration components of the brake actuators.
In the realization of regenerative brake systems, a motor vehicle is decelerated at a number of axles or, typically, also on all axles of the motor vehicle using friction brakes, each wheel of the corresponding axle being equipped with a friction brake. Depending on the motor vehicle being configured as an all-wheel vehicle or also as a vehicle driven on a single axle, the electric generator is generally connected to both vehicle axles or also to only one axle of the motor vehicle, so that generative brake torque needs to be generated through this axle only. The last-mentioned arrangement exhibits comparatively high efficiency because friction, which reduces the efficiency, is limited only to one of the axles. As an alternative of this concept, a so-called all-wheel concept wherein the generator connects to both axles, may also be put into practice. However, this alternative exhibits poorer efficiency due to higher friction so that it is rarely realized. However, in motor vehicles designed as all-wheel driven motor vehicles anyway, the generator, as has been described hereinabove, connects typically to both or to all of the axles of the all-wheel motor vehicle. Due to the configuration of the drive train in an all-wheel motor vehicle using a regenerative brake system, generally the brake torque is split up between front axle and rear axle in a predetermined fashion. In an all-wheel motor vehicle with two axles, the ratio of brake torques typically is approximately 50%/50% up to 30%/70% as regards the front axle in relation to the rear axle.
It is disadvantageous in a brake system configuration of this type that ideal brake force distribution is not given in at least proportionally regenerative braking. In particular in serial regenerative concepts, but also in parallel and mixed concepts, the result can be that one axle of the motor vehicle is decelerated proportionally at a higher rate than the other axles because a considerable part of the total brake torque is produced by the generator due to the assigned brake torque ratio at one axle. This effect is referred to as overbraking. When one axle is overbraked, the other axles or the other axle are underbraked. Thus, front-axle or rear-axle overbraking of the motor vehicle can occur in all-wheel drive motor vehicles with two axles, depending on how the brake torque distribution between front and rear axles of the generator is, while the respectively other axle is underbraked.
It is disadvantageous and problematic in overbraking that the overbraked wheels will become subject to slip more quickly than the other wheels. This may cause an ABS (Anti-Block System) brake operation being triggered by wheel slip. Consequently, the regenerative brake torque will be reduced completely so that after a short ABS-brake operation, normal braking, only by means of friction brakes, will occur again where brake force distribution is improved or more balanced, respectively. This action provokes an unstable brake feel with the driver. The braking effect as described, where ABS-braking occurs due to overbraking on one axle, is especially unfavorable and unpleasant for the driver in cornering maneuvers and at high deceleration values.